1. Field of the Invention
The present invention relates to an ATM (Asynchronous Transfer Mode) cell switching apparatus, and, more particularly, to the ATM cell switching apparatus which provides a separated route bypassing an ATM switch for control cells, including information for signaling and OAM (Operation, Administration and Maintenance) purposes, from a route passing through the ATM switch for ordinary user cells, including information of user data, to minimize degradation of traffic characteristics of the user cells which would be influenced by the control cells flowing in a same route.
2. Description of the Related Art
In the ATM technology, any type of information such as voice, low speed data and high speed data like moving picture is divided into fixed lengths of data, which are called cells and transferred asynchronously in the ATM network. The information includes not only ordinary user data as described, but also includes control data such as signaling information for controlling establishment and disconnection of a path for the user cells and other OAM information for operation, administration and maintenance purposes of the ATM network system concerned.
The user cells are transferred between user terminals and transferred in the ATM network transparently. However, the control cells are processed at an ATM node like the ATM cell switching apparatus for controlling the ATM network system operation. It is preferable to create a path for the control cells separate from a path for the user cells in the ATM cell switching apparatus to avoid the control cells giving any influences to the user cell transferred and switched in the ATM cell switching apparatus.
Conventionally, there are several technical disclosures which provide a separate path without passing through the ATM switch for the purpose of separation of user cells and the control cells, or other purposes.
Japanese Patent Application Laid-open Publication No. Hei 4-310033 discloses a technology which provides dedicated ports for the control cells coming into the ATM switch and doing out from the ATM switch as shown in FIG. 1(A). In this disclosure, in order to perform processing of the control cells, a control cell processing circuit (not shown) or a control cell distribution/insertion circuit (not shown) is connected to the dedicated port of the ATM switch. The control cells coming into the ATM switch from the dedicated port I-0 are distributed to each of output ports through the ATM switch as shown in FIG. 1(B), and the control cells coming into the ATM switch from each of incoming ports are routed to the dedicated port O-0 for out going as shown in FIG. 1(C). In any case, in connection with the control cells, the control cells are subjected to be switched in the same manner as the user cells.
According to this technology, the dedicated port is occupied by the control cell processing circuit, so that it is not preferable because the number of effective highway in the ATM switch is decreased. Moreover, when the cell distribution/insertion circuit is connected to the dedicated port, there arises a problem that the quality of the user cell deteriorates when the speed of the control cell increases.
Furthermore, Japanese Patent Application Laid-open Publication No. Hei 2-224548 teaches a provision of a separate path without passing through the ATM switch for user cells of a predetermined type of call which is considered as important as shown in FIG. 2. In this technology, the separate path is not provided for the control cell, and the separated path does not have a function for any processing of control cells.
Moreover, Japanese Patent Application Laid-open Publication No. Hei 5-48639 teaches that control cells coming from out side of the ATM switch are separated and format converted at a line interface unit, and transferred to a control cell processing unit through a different path from a path to the ATM switch as shown in FIG. 3. However, this technology teaches the processing of the received control cells only, and a relatively large amount of circuit unit is required as the line interface for each highway.
In any of the above-mentioned technologies, when the traffic amount of the control cell is increased during the switching operation at the ATM switch, it causes degradation of disposal quality of user cells (when requesting a call establishment, the user declares whether or not to permit disposal of cells, i.e. data, and the cell quality is determined by the degree of disposal) and delay quality (delay time of voice or moving picture from transmitting user terminal to receiving user terminal).